Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellowness, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Therefore, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, the most representative method is a method using an episulfide compound as described in Patent Documents 1-3.
The episulfide compound is obtained by producing a sulfur-containing epoxy compound and then sulfidating the compound. As methods for producing a sulfur-containing epoxy compound, the production methods described in Patent Documents 4 and 5 have been proposed, and the yield and transparency of resin obtained by curing an episulfide compound obtained by sulfidation have been improved. Patent Document 6 also describes a method for producing a sulfur-containing epoxy compound.